Solid-state heating-cooling zone control system

ABSTRACT

20 A multizone, heating-cooling control circuit implemented with solid-state components rather than conventional relays which heretofore have been commonly used. The control circuit is designed for use in a forced air system wherein each zone is provided with dampers and motors for controlling the positioning of the dampers in accordance with the heating or cooling demand detected by a thermostat in each of the zones. A plurality of power semiconductor devices (gate-controlled triacs) are employed to connect the windings of the zone control motors, the furnace fan, the furnace gas valve and the cooling compressor across an alternating current supply. Diode-transistor logic circuits are employed to generate the gating or control signal for the triacs in accordance with the settings of the system control switch and the temperature in one or more zones.

United States Patent AttorneyBurd, Braddock & Bartz ABSTRACT: 20 Amultizone, heating-cooling control circuit implemented with solid-statecomponents rather than conventional relays which heretofore have beencommonly used. The control circuit is designed for use in a forced airsystem wherein each zone is provided with dampers and motors forcontrolling the positioning of the dampers in accordance with theheating or cooling demand detected by a thermostat in each of the zones.A plurality of power semiconductor devices (gate-controlled triacs) areemployed to connect the windings of the zone control motors, the furnacefan, the furnace gas valve and the cooling compressor across analternating current supply. Diode-transistor logic circuits are employedto generate the gating or control signal for the triacs in accordancewith the settings of the system control switch and the temperature inone or more zones.

SOLID-STATE HEATING-COOLING ZONE CONTROL SYSTEM BACKGROUND OF THEINVENTION Many forms of furnace control circuits are described in theprior art. In these prior art arrangements, electromechanical switchingmechanisms such as electrical relays are employed to interconnectdifferent components of the system with a power supply in accordancewith control signals provided by one or more room thermostats. Thepresent invention relates to such heating/cooling control systems but isan improvement thereover in that solid-state electronic switches areemployed in place of the electromechanical relays. This offers distinctadvantages in terms of reliability, service life, ease of maintenanceand cost. In the preferred embodiment of the invention gate-controlledtriacs, which are functionally bidirectional semiconductor switchingdevices, are employed in place of relays found in prior art systems.Triacs elements are commercially available. The triac was firstdescribed in a paper appearing in the Journal of Applied Physics, Vol.30, number 1 1, dated Nov. 1959, entitled Two Terminal Asymmetrical andSymmetrical Silicon Negative Resistance Switches by R. W. Aldrich and N.l-Iolonyak, .Ir., and further information concerning their constructionand mode of operation can be obtained from that source.

Control over the firing of the triacs is determined by a noveldiode-transistor logic network. The network receives input signals frommaster and slave thermostats located in the building whose environmentaltemperature is to be controlled. The logic circuit combines thesesignals to provide the proper triac gating signals to permit conductionof the triacs and proper energization of the zone control motors, thefurnace fan, the furnace gas valve or the air conditioning compressor.

It is accordingly the principal object of this invention to provide anovel multizone heating/cooling system control network.

It is another object of the invention to provide a novel control networkfor a multizone heating/cooling system wherein solid state componentsare used throughout.

These and other objects, features, and attendant advantages of theinvention would be appreciated more readily as the same become betterunderstood by reference of the following detailed description whenconsidered in connection with the accompanying drawings in which:

FIGS. la, 1b and 1c illustrate by means of a schematic diagram thepreferred embodiment of the invention; and

FIG. 2 illustrates the manner in which the drawings of FIGS. through 1care arranged to provide a composite view.

Before describing the operation of the circuit of FIG. I, considerationwill first be given its construction.

Shown at the bottom of FIG. la is a schematic representation of a masterthermostat. In the preferred embodiment, the master thermostat maycomprise a Honeywell Type T87F unit having a Type 0539A subbase whilethe slave thermostats (FIGS. lb and 1c) comprise only the Type T87Funit. The thermostat is shown to include a bimetallic element 2 whichwhen subjected to a change in temperature moves to the right or leftdepending upon whether the temperature is falling or rising,respectively. Associated with the bimetallic strip 2 are a pair ofcontacts 4 and 6 which may be termed the heating contact and the coolingcontact respectively. Included in the master thermostat (FIG. la) is amanually operated system control switch which, when set, detennineswhether the system will be in a heating or a cooling mode. The systemcontrol switch includes a first single pole three-position switchindicated generally by the numeral 8 and a three-position shortinglinkage type switch indicated generally by numeral 10. While in FIG. 1a,the switches 8 and 10 are shown as separate, in practice they are gangedand caused to operate by the same switch mechanism. The masterthermostat further includes another single pole, double throw switch 12which is the fan control" switch. The arm 14 of this switch is movablebetween a pair of contacts 16 and 18. When the arm 14 and contact 16 aremade, the fan is in a continuous run or on" condition. However, whenswitch arm 14 mates with contact 18, the fan is in an "automatic"condition as will be described further hereinbelow. A pair of heaters 20and 22 termed anticipators are included in the thermostat and serve tolimit room temperature overshoot in either the heating or the coolingmode.

At the bottom of FIGS. lb and 1c are shown the slave-type thermostatsemployed in the several zones comprising the system. These slavethermostats differ from the master thermostat in that they do notinclude a system control switch 8, 10, or a fan control switch 12. Allthat is included is the bimetal element 24, a heating contact 26, acooling contact 28, and the anticipators 30 and 32.

Referring now to FIG. lc, there is shown a power supply for the controlcircuit. This power supply includes a transformer 34 having a primarywinding 36 and a secondary winding 38. The primary winding is adapted tobe connected to a source of alternating current voltage at linepotential and the transformer 34 steps the voltage down to a desiredlevel or value. Also included in the power supply is a diode rectifierbridge including the semiconductor diodes CR2] through CR24. The bridgeacts in a conventional manner to provide full wave rectified DC voltagebetween the ground terminal 40 and the 13+ terminal 42. The B+ terminal42 of the power supply is connected by a bus 44 to a terminal Zl (FIG.la). The bimetal strip 2 of the master thermostat is connected toterminal Z1 as is the switch contact 16 of the fan switch 12. Aconductor 46 connects the switch arm 48 of the system control switch 8to the terminal Z1 also. Finally, the anticipator heater element 20 isconnected between the thermostat cooling contact 6 and the B+ terminalZl.

The ground terminal 40 is connected by a bus 50 to a first terminal of aplurality of bidirectional semiconductor switching elements or triacs Tlthrough T9. Triacs T1 and T2 are associated with zone 1 and respectivelydetermine whether the damper motor for that zone will close or open. Thetriac T3 controls the furnace gas valve. Triacs T4 and T5 control thedamper motor for zone 2. Triac T6 controls the air conditioningcompressor. Triac T7 controls the fan while triacs T8 and T9 control thedamper motor in zone 3. Of course, it is possible to add additionalzones into the system by adding additional damper motors and triacs inaccordance with the organization depicted in the drawings. To addadditional zones is felt to be well within the realm of ordinary skillin the art provided one is familiar with the teachings of the presentapplication.

The energy source for the damper motors comprises the transformer 52which has a primary winding 54 adapted to be connected to a source ofline potential and a secondary winding 56 connected between the groundbus 50 by a conductor 58 and a common bus 60 by a conductor 62. Thedamper control motors for each of the zones is preferably a reversiblealternating current motor and while not shown in the figures, would beconnected to the terminals ll, L1, and P1 in such a fashion that whentriac T1 is conducting current would flow from the secondary winding 56through conductor 62 and bus 60 through the motor winding, terminal P1,a conductor 64, triac TI to ground bus 50 to which the other side of thesecondary winding 56 is connected by way of conductor 58. The currentflow through the motor winding along this path causes the motor to turnin a direction to close the damper, with suitable limit switch meansbeing utilized to control the extent of drive.

When triac T2 is conducting, alternating current flows from thetransformer secondary winding 56 to terminal I] and from there throughthe motor winding to terminal L1 and through triac T2 and ground bus 50back to the other side of the winding 56. The current flowing throughthe motor winding connected between tenninals I1 and L1 causes thedamper motor to turn in a direction causing the damper to open. Whilethe hookup of the damper motor for zone I has been described in detail,it is felt unnecessary to repeat this explanation for the damper motorsin zones 2 and 3 for they are connected in a similar fashion to theterminals L2, P2, and l2, and L3, P3, and [3.

As mentioned above, the triac T3 controls the energization of thefurnace gas valve. More specifically, a transformer 66 having a primarywinding 68 connected to a source of line potential induces a voltage ofa desired value in the secondary winding 70 thereof. The gas valve 72 isconnected in a series circuit with the secondary winding 70, a terminalG1 and the triac T3 by a conductor 74. The other side of the triac T3 isconnected by conductor 76 to the ground bus 50 which is connected to theother side of secondary winding 70 by way of conductor 78 and terminalG2. Thus, when triac T3 is conducting, a low impedance path includingthe gas valve 72 is established across the secondary winding 70 oftransformer 66.

In much the same fashion, the triac T7 controls the energization of thefan relay 80. Specifically, the secondary winding 56 of transformer 52is connected by way of conductor 60 to one side of the relay 80. Theother terminal of relay 80 is connected to a terminal F1 which isconnected by conductors 82 and 84 to one side of the triac T7. Aconductor 86 joins the other side of triac T7 to the ground bus 50 whichis returned to the remaining terminal of the secondary winding 56 by wayof conductor 58. When triac T7 is conducting, current flows from thesecondary winding of transformer 52 through the relay 80 and triac T7 toenergize the fan relay.

The air conditioning compressor 88 is connected in a series circuit witha secondary winding 90 of a transformer 92 whose primary winding 94 isconnected to a source of line potential. Also connected in series withthe secondary winding 90 and air conditioning compressor motor 88, is aterminal C1, conductors 96 and 98, triac T6, a conductor 100, the groundbus 50 and conductor 78 which is, in turn, connected to the secondarywinding 90 by a conductor 102. When triac T6 is nonconducting, a highimpedance is presented to the fiow of current through this path so thatthe compressor will remain off. However, when the triac T6 is turned onby a suitable control signal applied to its gate electrode, a lowimpedance exists in the above-described series circuit and thecompressor motor will be energized.

Referring again to F 16. 1a, it will be seen that a contact 104 of themaster thermostat is connected to a terminal W1 while a contact 106 isconnected by a conductor 108 to a terminal of heater 22. Furthermore, acontact 110 of switch is connected to the junction 112 between thecooling contact 6 and the anticipator heater 20. A cooling contact 114is connected to a terminal Y1 and also to the auto switch contact 18 bymeans of a conductor 116.

The system function switch 8 has its heating contact 118 connected to aterminal B and to terminals W2 and W3 by a conductor 120. The coolingcontact 122 of switch 8 is connected to a terminal Y2 and a terminal Y3by a conductor 124.

The switch arm 14 of the fan control switch 12 is connected to aterminal G. A conductor 126 connects the terminal W2 to the seriescombination of the heater 32 and the heating contact 26 of the slavethermostat for zone 2. ln a similar fashion, a conductor 128 connectsthe terminal W3 to the series combination of heating contact 26 andanticipator heater 32 of the thermostat for zone 3. The cooling contact28 of zones 2 and 3 are respectively connected to the terminals Y2 andY3 by a conductor 130 and 132. The bimetal elements of the thermostatsfor zones 2 and 3 are connected to terminals Z2 and Z3 respectively.

Now that the external connections to the control circuit of thisinvention have been described, consideration will be given to theorganization of the diode-transistor logic circuits used to develop thegating signals for the plurality of triacs used in the system.

Referring to FIG. la, a conductor 134 connects to the ground bus 50 atjunction 136. A pair of NPN transistors 02 and 03 have their emitterelectrodes connected to the grounded conductor 134 by conductors 138 and140. The

base of transistor O2 is coupled through a resistor R9 and a diode CR25to the terminal Y1. A conductor 140 connects the terminal W1 to thejunction 142 between resistor R9 and diode CR25 and a pair of resistorsR1 and R2 are connected between this junction and the ground conductor134. The junction 142 is also connected by means of conductors 144 and146 and a diode CR26 to a junction 148. Junction 148, in turn, iscoupled through a resistor R13 to the gate electrode 150 of triac T2.

The collector electrode of transistor 02 is connected to a junction 152to which a pair of diodes CR5 and CR6 are connected. The other terminalof diode CR6 is connected to a junction 154 between a first terminal ofa resistor R14 and an additional diode CR7. The remaining terminal ofresistor R14 is connected to the B+ bus 44 and the other terminal of thediode CR7 is coupled through a resistor R32 to the base electrode oftransistor 03.

A resistor R12 is connected between the bus 44 and the remainingterminal of the diode CR5. This junction point is identified by numeral156. The junction 156 is coupled through a diode CRIS to the gateelectrode 158 of triac T1.

The collector electrode of transistor 03 is connected by a conductor 160to the anode electrode of a diode CR4 whose cathode is connected to thejunction 156.

The B+ bus 44 is coupled by means of a conductor 162 and a resistor R11to a junction point 164 to which the cathode electrodes of diodes CR1,CR2 and CR3 are commonly connected. The anode electrode of diode CR1 isconnected by a conductor 166 to the terminal B (FIG. 1a). The anodeelectrode of diode CR2 is connected to the conductor 160 at junction168. The anode electrode of diode CR3 is connected by a conductor 170 tothe gate electrode 172 of triac T3. A resistor R7 is connected betweenthe conductor 166 and the ground bus 50 by a conductor 174.

Referring now to FIG. 1b, it will be seen that the terminal Z2 isconnected by conductors 176 and 178 to a junction 180. Connected betweenthe junction 180 and a ground conductor 182 are a pair of paralleledresistors R3 and R4. Also connected to the grounded conductor 182 is theemitter electrode 184 of a transistor 01 whose base is coupled through aresistor R10 to the junction 180. The terminal Z2 is connected by meansof conductor 176 and a diode CR28 and a resistor R15 to the gateelectrode 186 of triac T5. The collector electrode of transistor O1 isconnected to a junction 188 to which the anode electrodes of a pair ofdiodes CR9 and CR10 are connected. The cathode of CR10 is connected to ajunction 190 to which the base electrode of a transistor O6 is directlyconnected. The cathode electrode of diode CR9 is coupled through a diodeCR16 to the gate electrode 192 of triac T4. The emitter electrode of thetransistor 06 is connected by a conductor 1 94 to a junction point 196on a bus 198. The bus 198 connects to the cathode of a diode CR27 (FIG.1a) whose anode is coupled to the junction point 148.

The collector electrode of transistor O6 is connected by a conductor 200to the B+ bus 44. A resistor R16 is bridged between the. conductor 200and the junction formed by the common connection of the cathodes ofdiodes CR8, CR9 and CR16.

Continuing on with a detailed description of the manner in which thecomponents forming the control circuit are interconnected, a conductor202 connects the terminal G to which the fan switch 12 is connectedthrough a diode CR30 to a junction 204. The junction 204 is coupledthrough a resistor R18 to the gate electrode 206 of the triac T7. Theterminal Y3 is connected by a conductor 208 to a junction 210. Coupledbetween junction 210 and the ground bus 40 is a resistor R8. Alsocoupled to the junction 210 is the anode of a diode CR12 whose cathodeis connected in common with the cathode of a diode CR1 1 to ajunction212. The anode of diode CR11 connects to the collector of transistor Q3by way of the conductor bus 160. Junction 212 is coupled to the baseelectrode of a transistor 04 through a diode CR13. The collectorelectrode of the transistor O4 is connected by a conductor 214 to thepositive bus 44. The emitter electrode of transistor O4 is coupledthrough a resistor R19 to the gate electrode 216 of triac T6 and througha diode CR14 and the resistor R18 to the gate electrode 206 of triac T7.

The positive bus 44 is coupled by a conductor 218 and a resistor R21 toa junction 220. Thisjunction is coupled through a diode CR20 to the gateelectrode of the triac T8. A diode CRIS has its cathode connected to thejunction 220 and its anode connected to the conductor bus 160.

The terminal Z3 (FIG. is connected to a junction 222. This junction iscoupled through a resistor R20 to the base electrode of a NPN transistorQ5, whose emitter electrode is connected to the ground bus 50 by way ofa conductor 224. Connected between conductor 224 and the junction 222are a pair of resistors R5 and R6. The collector electrode of transistor05 is coupled through a diode CR19 to the junction 220. Junction 220, inturn, is coupled through diode CR20 to the gate electrode of triac T8.The collector electrode of transistor 05 is also coupled through a diodeCR17 to the junction 190. Finally, the terminal Z3 is connected througha diode CR32 and a resistor R31 to the gate electrode 226 of triac T9.The gate electrode 226 is also coupled through a diode CR31 to thejunction 196 connected to the emitter of transistor Q6.

Now that the detailed organization of the preferred embodiment has beendescribed in detail, consideration will be given to its operation.

OPERATION-HEATING MODE As an initial condition, let it be assumed thatthe dampers are all in the open position, the system control switch 8and 10 is in the heat position but that no zones are calling for heat.With alternating current connected to the primary windings of thetransformers 34, 52, 66, and 92, the system will be ready to function.Specifically, a rectified DC voltage will be impressed across theterminals 40 and 42 such that a positive voltage will appear on the B+bus 44. Because under the assumed conditions transistors 01, Q2, and Q5are nonconducting, this positive signal is applied through the resistorR14 to the junction 154 and through diode CR7 and R32 to maintaintransistor 03 conductive. With transistor Q3 conducting, groundpotential will be applied by way of conductor 140 and theemitter-to-collector path of transistor 03, the conductor 160 tothejunction 168. This ground or low signal will be coupled through diodeCR2 and CR3 to the gate electrode 172 of triac T3 to insure that triacT3 does not conduct. With triac T3 nonconducting, a high impedance isincluded in the path containing the gas valve and the gas remains off.Specifically, insufficient current will flow because of the highimpedance presented by triac T3 to allow energization of the gas valve72. Under the assumed conditions, this is proper operation since it hasbeen assumed that no zone is calling for heat.

The positive signal appearing at junction 154 to maintain transistor 03conducting is also applied to the junction 190 so that transistor Q6will also be conducting at this time. With transistor 06 in itsconductive state, the B+ voltage appearing on bus 44 will be applied byway of conductor 200 and the collector-to-emitter path of transistor 06and the conductor 194 to junction 196 on conductor 198. This positivesignal is coupled by diode CR27 and resistor R13 to the gate electrode150 of triac T2. Similarly, this positive signal will be coupled throughdiode CR29 and resistor R to the gate electrode of triac T5. Further,the positive signal on the conductor 198 which exists by virtue of thefact that transistor 06 is conducting also is coupled through diode CR31and resistor R31 to the gate electrode 226 of triac T9. With the gateelectrodes of triacs T2, T5 and T9 all positively biased, these triacswill be conductive and, as mentioned earlier, the reversible AC dampercontrol motors will be operated to maintain the dampers in the openposition.

Next it is to be assumed that the fan switch 12 is moved to the onposition, i.e., switch arm 14 is mated with contact 16.

With this assumption prevailing, it will be seen that a positive voltageappearing on bus 44 will be coupled through the terminal Z1 and throughthe fan switch 12 and terminal G and conductor 202 and diode CR30 to thejunction 204. This positive signal is coupled through resistor R18 tothe gate electrode 206 of the triac T7. The effect of this bias voltageis to force the triac into its conducting state. As a result, the fanrelay will be effectively connected directly across the secondarywinding 56 of transformer 52, all as previously described.

Next, it is to be assumed that the fan control switch is moved to theautomatic (auto") position, i.e., contact arm 14 mating with contact 18and further that the thermostat 2 in zone 1 is calling for heat. Whenthe bimetal strip 2 mates with the heating contact 4, a positive voltagefrom the B+ bus 44 will be applied through terminal Z1, the bimetalstrip 2, the heater element 22, conductor 108, the switch arm 10bridging contacts 104 and 106 such that terminal W1 is at approximatelyB+ potential. This potential is applied to the junction 142 by way ofconductor and causes the transistor O2 to be forward biased and turnedon. With transistor Q2 conducting, ground potential appearing onconductor 134 will be coupled through the transistor to the junction152. A current flows through R14, CR6, and Q2 with the voltage dropacross R14 causing a low potential at junction 154. This low signalcoupled through diode CR7 and resistor R32 to turn off the transistorQ3. As a result, ground potential will be removed from the conductor 160and, instead, a high potential will be applied thereto by way of theresistor R17 and the diode CRll. With the conductor 160 at a highpotential, this signal will be coupled through the diodes CR2 and CR3 tothe gate electrode 172 of triac T3, thus causing the triac T3 to assumeits low impedance state. The alternating current voltage induced in thesecondary winding 70 will flow through the gas valve 72 throughconductor 74 through the triac T3, through conductors 50 and 78 toenergize the gas valve so that the burner can be ignited.

The fan switch has been assumed to be in the auto position. Becausetransistor O2 is conducting, the B+ voltage on bus 44 will be droppedacross R12 and the junction 156 will be at a low potential andaccordingly triac T1 will be in its high impedance state. Because of theB+ voltage appearing at terminal W1 under the assumed conditions, thishigh signal will be coupled through the diode CR26 and resistor R13 tothe gate electrode of triac T2. Hence, triac T2 will be fully conductiveand the damper motor for zone 1 will be maintained in its openedposition.

It has also been shown that transistor Q3 will be off at the time thatzone 1 calls for heat. With-transistor Q3 nonconducting, the conductorwill be at a relatively high potential and as a result, this high signalwill be coupled through diodes CR8 and CR16 to turn on triac T4. As hasbeen mentioned above, the energization of triac T4 causes a current toflow through the zone 2 damper motor in such a fashion so as to effect aclosure of the damper. Similarly, the high signal appearing on conductor160 is coupled through diodes CR18 and CR20 to the gate electrode oftriac T8. The efi'ect of this is to energize the winding of the dampermotor for zone 3 in such a fashion that the damper is closed. Hence, itcan be seen that under the assumed conditions, the gas valve will beturned on, zone 1 which is the zone which called for heat, will have itsdamper remain open while the dampers for zones 2 and 3 will be driven totheir closed position. The fan will be energized by the furnace fanrelay.

To insure that the zone 2 and 3 damper motors will remain closed, it isnecessary that triacs T5 and T9 nonconducting. Because transistor O2 isdriven on when zone 1 calls for heat, a low signal will be coupledthrough the transistor ()2 and the diode CR6 to the junction 154. Thispotential level also appears at the junction causing transistor 06 to benoncon ducting. With transistor Q6 nonconducting, and with diodes CR29and CR31 being coupled to the gate electrodes 186 and 226, triacs T5 andT9 remain off. i

HEATING MODE-ZONE 1 AND ZONE 3 DEMANDING HEAT Now let it be assumed thatwhile zone 1 continues to demand heat, that zone 3 also demands heat.Under this assumption, when the bimetal switch 24 in zone 3 thermostatmoves against the heating contact 26, a positive voltage from the bus 44will be coupled from terminal Z1 by way of conductor 46, system switch8, conductor 120, conductor 128, anticipator heater 32, the switchcontact 26 so as to appear at terminal Z3. The high signal coupledthrough diode CR32 and resistor R31 is applied to the gate electrode 226of triac T9 causing triac T9 to conduct. The effect of this is to couplethe motor winding between terminal L3 and terminal 13 directly acrossthe secondary winding 56 of the transformer 52. Energization of thiswinding will cause the damper motor to open the damper in zone 3. Thehigh potential at terminal Z3 also causes transistor O5 to becomeconductive, thereby grounding the anodes of diodes CR19 and CR17 by wayof conductor 224 and the emitter-to-collector path of transistor Q5. Thelow potential appearing at junction 220 is coupled by way of diode CR20to the gate electrode of triac T8. This insures that triac T8 willremain off so that no current will flow through the winding which tendsto rotate the damper motor in a direction to close the damper.

Transistor 06 remains off because of the low potential level alongconductor 134, conductor 138, transistor Q2 and diode CR6 to thejunction 190. With transistor 06 nonconducting, the signal appearing onconductor 194 will remain low and this low potential is coupled by diodeCR29 and resistor R15 to the gate electrode 186 of triac T5. Therefore,triac T5 remains nonconducting and the damper motor for zone 2 remainsin its closed condition. At the same time, transistor 01 isnonconducting such that the junction between diodes CR8, CR9 and R16remains relatively high, and this B+ signal is coupled through resistorR16 and the diode CR16 to the gate electrode 192 of triac T4. Thepositive signal at electrode 192 insures that triac T4 will beconducting and the damper motor for zone 2 will have its windingsenergized so as to urge the damper to its closed position.

With the operation as it has been thus far described, it is believedunnecessary to explain the details of operation resulting when thethermostat for zone 2 should sense the need for heat. The operation istotally similar to the operation described in connection with zone 3 andneed not be repeated.

COOLING MODE To convert the control circuit of the present invention tooperate in controlling an air conditioning compressor, the system switch8, 10 is manually moved to the cool" position, i.e., with switch arm 48mating with contact 122 and the linkage moved to bridge contact 110 tocontact 114. Under these circumstances, the circuit will remain dormantuntil one of the zones calls for cooling. As an initial condition, letit be assumed that the dampers in all three zones are in their closedposition and that no zone is calling for cooling. With these assumptionsexisting, no potential will be applied to the base electrode oftransistor Q2 and it will be nonconducting. As a result there will be novoltage drop across the resistor 14 due to the fact that zones 2 and 3are also satisfied such that transistors 01 and 05 are nonconducting.With no voltage drop across resistor R14, junction 154 will be high andtransistor Q3 will be forward biased and will be heavily conductive.When transistor O3 is conducting, the bus 160 is effectively connectedto ground potential by way of transistor Q3 and the conductor 134connected to ground bus 50 at junction 136.

Because bus 160 is grounded, current will flow from the B+ bus 44through resistor R12 and diode CR4 so that junction 156 will be at arelatively low potential. A low potential at this terminal reversebiases the triac T1 which disables the triac and removes theenergization from the damper motor winding which tends to close the zone1 damper.

As was mentioned above, with transistor Q2 nonconducting, the junction154 will be at a relatively high potential as will the junction 190which is connected to junction 154 by conductor 191. The high potentialat junction 190 causes transistor 06 to be forward biased andconducting. Hence, a positive potential from the B+ bus 44 will beapplied by way of conductors 200 and 194 and 198 to the diodes CR27,CR29, and CR31. The positive signal is coupled through these diodes andthe resistors R13, R15, and R31 to the gate electrodes of triacs T2, T5and T9. Hence, the zone control motors for zones 1, 2 and 3 will havetheir windings energized by alternating current supplied by thetransformer 52 so as to cause the dampers to be moved to an openposition. Thus, as soon as the master thermostat has the system functionswitch moved to the cool" position the dampers for each of the zoneswill automatically be opened preparatory to the initiation of the airconditioning compressor if no zone is calling for cooling.

With transistor Q3 conducting, a current path is established from thepositive bus 44 through conductor 162 and resistor R11 and through diodeCR2 and conductor to the grounded conductor 134. This maintains thejunction point 164 a a low potential and reverse biases the triac T3.Hence, the furnace gas valve will be disabled as it should be when theheating/cooling system is operating in a cooling mode. in a similarfashion, a current from the B+ supply through conductor 214 and resistorR17 and through diode CRll to the presently grounded bus 160 maintainsjunction 212 at a low potential such that transistor O4 isnonconducting. With transistor Q4 nonconducting, the triac T6 isnonconducting and the air conditioning compressor is accordinglyisolated from the secondary winding of transformer 92. Similarly, withtransistor Q4 nonconducting, no base drive current for triac T7 will beapplied through diode CR14. Assuming that the fan switch 12 is in itsmidposition, no base drive current for the triac T7 will be availableand hence it will remain nonconducting. As such, the fan relay 80 isisolated from the alternating current supplied by the secondary winding56 of transformer From the description thus far, it can be seen thatwhen the system function switch is in the cool" position with all of thezones satisfied, the control circuits will operate the triacs in such afashion that the dampers for all three zones will be opened, the gasvalve will be disabled, the compressor will be disabled, and the fanwill be disabled.

if it is next assumed that the fan switch 12 is moved to its on positionwith switch arm 14 mating with contact 16, the

current will flow from the B+ supply through bus 44, terminal Z1, theswitch contacts 16-14 to tenninal G. From there, the current will flowthrough conductor 202 and diode CR30 to junction 204. This provides therequisite base drive current for triac T7 to drive it to its conductivestate. With triac T7 conducting, the fan relay 80 will be connectedacross the secondary winding 56 of transformer 52 and will be energized.However, if the switch 14 is now moved to the auto" position, this flowof current will be interrupted and the fan will be deenergized.

To further understand the operation of the circuit of this invention,let it be assumed that the thermostat in zone 2 (FIG. 1b) calls forcooling. Under these circumstances, the bimetal element 24 will matewith the cooling contact 28 in zone 2. As a result, current will flowfrom the positive bus 44 through terminal Z1 (FIG. 1a) through conductor46 through contacts 48 and 122, through conductor 124 and 130, andthrough the bimetal element 24 and conductor 176 to forward bias thetransistor Q1 rendering it conductive. With transistor Q1 conducting,junction 188 will be at a low potential and a current will flow throughthe positive bus 44, resistor R14 (FIG. 1a) and conductor 191 tojunction 190 and from here, through diode CR10 and through transistor O1to the grounded conductor 182. The voltage drop across resistor R14causes junction 190 to assume a low potential and transistor 06 isturned off. With transistor 06 turned off, the positive voltage isremoved from the diodes CR27, CR29 and CR31. it is to be recalled thatit was a positive signal applied to these last-mentioned diodes thatcaused the damper motors in all of the zones to assume an open position.The positive signal appearing at terminal Z2 when zone 2 is calling forcooling is coupled through diode CR28 and resistor R15 to turn on thetriac T5. Since triac T5 is conducting, the damper motor in zone 2 willcontinue to hold the damper in an open position.

In certain instances, it may be desirable to slave certain damper motorstogether to insure adequate airflow over the evaporator coils during thecooling operation whenever only one zone is calling for cooling. Also,the system may be modified so as to permit the compressor and fan to beenergized only when more than one zone is calling for cooling. While itmay appear strange that the air conditioning compressor and fan will notcome on even though one of the zones is calling for cooling, the reasonfor this is that with only one zone operating, insufficient air passesover the evaporator coils in the air conditioning unit and icing canoccur.

Now let it be assumed that zone 1 calls for cooling. As such, thebimetal element 2 will move against the cool" contact 6 in thethermostat for zone 1 (FIG. la) and the B+ voltage is applied toterminal Y1. Current now flows through diodes CR25, CR26 and resistorR13 to forward bias the triac T2 to insure that the damper for zone 1 isopened. The positive signal at terminal Y1 also forward biasestransistor Q2 turning it on. With transistor Q2 conducting, junction 152is at ground potential and a current now flows from the positive bus 44through resistor R14, junction 154, diode CR6 to the grounded junction152. This lowers the potential at junction 154 causing transistor O3 tobe turned off. The current from the positive bus 44 through resistor R12and diode CR5 causes the potential at junction 156 to drop such that areverse bias is applied to triac TI. This has the effect of disablingthe winding of the zone 1 damper motor which would effect closure of thedamper.

Because of the foregoing detailed explanation, it is felt to be wellwithin the realm of ordinary skill in the art for one to analyze theoperation of the circuit of this invention when it is assumed that zone3 is calling for cooling. Hence, it is deemed unnecessary to explainthis mode of operation in detail since it is substantially the same asalready set forth in connection with the operation of zone 2.

Having described the preferred embodiment of the present invention, itis believed obvious that other modifications and variations of thepresent invention are possible in light of the above teachings.

What is claimed is:

1. In a multizone forced air heating/cooling system of the typeincluding duct work leading to a plurality of zones each havingtemperature-responsive switching means located therein, a gas-firedfurnace having an electrically operated gas valve, a relay-controlledfan motor, an air conditioning unit including a motor-driven compressor,and a plurality of motordriven dampers in said duct work for controllingthe flow of air to said zones, a solid-state control system comprising:

a. A first, second and third semiconductor gate controlled switchingdevices each having a pair of load terminals and a gate terminal;

b. means connecting said pair of load terminals of said first, secondand third switching devices individually in series circuit with a sourceof alternating current and said gas valve, said relay and saidmotor-driven compressor;

c. a plurality of pairs of semiconductor gate controlled switchingdevices each having a pair of load terminals and a gate terminal, saidload terminals connected in series circuit with said source ofalternating current and said plurality of damper motors, such that whenone of said pair of switching devices is conducting said motors moves ina first direction and when the other of said pair of switching devicesis conducting, said motors move in an opposite direction; an

d. means connected to said temperature-responsive switching means ineach of said zones and coupled to said gate electrodes on said first,second third and said plurality of pairs of semiconductorgate-controlled switching devices for controlling the conduction thereofin accordance with the setting of said temperature-responsive switchingmeans.

2. Apparatus as in claim 1 wherein said last-mentioned means comprisessemiconductor current control devices each having a control electrodeand a pair of output electrodes associated with each of said zones;

means connecting the temperature-sensitive switching means for each ofsaid zones to the control electrode of the semiconductor current controlmeans associated with that zone; and

means connecting the pair of output electrodes between said source ofdirect current potential and the gate electrode of said pairs ofsemiconductor gate-controlled switching devices.

3. Apparatus as in claim 2 wherein said semiconductor current controlmeans are transistors and where said last-named means includes diodelogic gating means.

4. Apparatus as in claim 1 wherein said semiconductor gatecontrolledswitching devices are triacs.

5. Apparatus as in claim 1 wherein said temperature-sensitive switchingmeans for each of said zones comprises a bimetal switch thermostat.

1. In a multizone forced air heating/cooling system of the typeincluding duct work leading to a plurality of zones each havingtemperature-responsive switching means located therein, a gasfiredfurnace having an electrically operated gas valve, a relaycontrolled fanmotor, an air conditioning unit including a motordriven compressor, anda plurality of motor-driven dampers in said duct work for controllingthe flow of air to said zones, a solid-state control system comprising:a. A first, second and third semiconductor gate controlled switchingdevices each having a pair of load terminals and a gate terminal; b.means connecting said pair of load terminals of said first, second andthird switching devices individually in series circuit with a source ofalternating current and said gas valve, said relay and said motor-drivencompressor; c. a plurality of pairs of semiconductor gate controlledswitching devices each having a pair of load terminals and a gateterminal, said load terminals connected in series circuit with saidsource of alternating current and said plurality of damper motors, suchthat when one of said pair of switching devices is conducting saidmotors moves in a first direction and when the other of said pair ofswitching devices is conducting, said motors move in an oppositedirection; an d. means connected to said temperature-responsiveswitching means in each of said zones and coupled to said gateelectrodes on said first, second third and said plurality of pairs ofsemiconductor gate controlLed switching devices for controlling theconduction thereof in accordance with the setting of saidtemperature-responsive switching means.
 2. Apparatus as in claim 1wherein said last-mentioned means comprises semiconductor currentcontrol devices each having a control electrode and a pair of outputelectrodes associated with each of said zones; means connecting thetemperature-sensitive switching means for each of said zones to thecontrol electrode of the semiconductor current control means associatedwith that zone; and means connecting the pair of output electrodesbetween said source of direct current potential and the gate electrodeof said pairs of semiconductor gate-controlled switching devices. 3.Apparatus as in claim 2 wherein said semiconductor current control meansare transistors and where said last-named means includes diode logicgating means.
 4. Apparatus as in claim 1 wherein said semiconductorgate-controlled switching devices are triacs.
 5. Apparatus as in claim 1wherein said temperature-sensitive switching means for each of saidzones comprises a bimetal switch thermostat.